Tough one. I was going along with the posts about how irresponsible FIL was being, and then I saw DavidH's post and now I'm not sure... Sounds like DH is going to talk to FIL about it, so maybe that will provide further clarity (for the OP, if not us).
Honestly if this were my own parents I would know them well enough to know whether I ought to say something to my dad or to my mom, or not at all, and I think my dad would listen to me if I approached it the right way. But with anyone else it would be really, really difficult to know what to do. I would not want to be a party to something that would be hurtful to one half of the couple, but on the other hand, I wouldn't want to make too many assumptions and involve myself in the couple's relationship.
If I chose not to get involved directly, I think I would give them a generous monetary gift as an "anniversary present" and hope MIL didn't earmark it for anything before the credit card bill arrived.
It reminds me a little bit of how my dad likes to swoop in and pay for everything for my mom. But they have joint finances, so the money comes out of the same account no matter who actually handled the credit card, and my mom doesn't have any aversion to handling the transaction herself. Yet my dad gets "credit" in certain circles for being gentlemanly or generous, and my mom is kind of rolling her eyes in the background. You can bet there would be fireworks if my dad tried to pull this with something so costly...